The invention relates to an undercutting tool for producing undercuts in pilot bores in masonry, concrete, rock and the like, having a rotary tool barrel which can be inserted into the bore that is to be undercut, at least one cutting tip being disposed in the bore-internal end portion of the barrel such that its cutting edge can be displaced gradually and radially outwardly into the wall of the pilot bore when the barrel is placed in the latter, the cutting tip being disposed on a cutter in the form of a two-armed lever which is fulcrumed in a longitudinal slot provided in the bore-internal end portion of the barrel and is capable of being rocked by the bore-internal end of a cutter feed driver which is mounted for longitudinal displacement by a certain amount in the barrel but held co-rotationally therewith, so that, by pushing the cutter feed driver into the barrel, the cutter can be shifted from a position in which the lever is retracted into the slot to a position in which the cutter projects from the barrel, a spring being provided in the undercutting tool, which biases the cutter toward the retracted position.
In this known bore undercutting tool (German Offenlegungsschrift No. 2,731,901, FIG. 2), which has been tested and found practical, the spring biasing the cutter toward the retracted position directly engages the lever arm of the cutter at its rearward end, i.e., the end pointing toward the orifice of the bore, being disposed under bias between the back of this lever arm and the opposite wall of the slot containing the cutter. Since the amount of the interstice available for the spring is, however, very limited, the biasing force exerted by the spring is also limited. After the known tool has been operated, the dust cut from the bore wall clogs the recess in the cutter and in some cases it may happen that the retraction of the cutter into the slot afterwards is prevented or the force of the spring is not sufficient to withdraw the cutter all the way into the slot, so that additional manipulations are necessary in order to remove the undercutter from the bore. It has furthermore been found that the time required for undercutting in high-strength concretes is relatively long, and this is to be attributed to the fact that the thrust exerted by the tool feed driver, which in the known tool is applied through a ramp on the end of the cutter feed driver to the lever arm on the bore orifice end of the cutter, engages the lever arm with a leverage which diminishes as the cutter is advanced into the material, i.e., the percentage of the thrust on the cutter feed driver which is transformed to the thrust of the cutter diminishes as the depth of the undercut increases.
Accordingly, it is the object of the invention to improve the cutting efficiency of the undercutting tool to such an extent that it will be possible to produce undercuts in bores in high-strength concretes within a relatively short time, and still avoid any impairment of the operation of the undercutter by the dust produced by the cutting action.
Setting out from an undercutting tool of the kind mentioned above, this object is achieved in accordance with the invention in that the back edge of the inner arm of the cutter, which bears the cutting tip is at an angle to the direction of movement of the cutter feed driver, and is engaged by a cutter feed cam formed on the inner end of the cutter feed driver, and this cam rocks the cutter out of the slot in the barrel with a leverage which becomes increasingly greater as the cutter feed driver is moved relatively to the barrel into the interior of the bore. Thus, by the shifting of the point at which the feed driver engages the back edge of the inner arm of the cutter an improvement of the transmission of force is achieved, resulting in an improvement of the cutting action as the cutting process progresses.
In a preferred further development of the invention, the back edge of the outer arm of the cutter is at an angle opposite that of the inner arm, and it is engaged by the cutter feed cam when the driver is drawn out of the bore interior relative to the barrel. The withdrawal of the cutter feed driver after the undercut has been completed then positively produces a retraction of the cutter into the slot in the barrel. There is consequently no longer any need to provide in the slot in the shaft a spring for the retraction of the cutter. Since the operation of the tool, however, is simplified, when the cutter is automatically retracted into the slot upon the completion of the undercut, the invention is further improved by providing on the portion of the cutter feed driver extending from the barrel a coil spring compressively biased against the barrel at one end and against the cutter feed driver at the other; this spring automatically draws the cutter feed driver back out of the barrel when the operator stops exerting pressure on it.
It is desirable for the coil spring to thrust at one end against the bore orifice end of the barrel, and at the other end against a collar provided on the cutter feed driver, and it may be desirable to enable the collar to be affixed at selectable axial positions on the cutter feed driver projecting from the barrel, since in this manner the bias of the coil spring will be variable.
At the same time it is desirable to dispose the coil spring within a sleeve attached to the end of the barrel, and for this sleeve to be engaged in a longitudinally displaceable manner by the end of the collar facing the bore. Thus the coil spring will be completely encapsulated within the sleeve.
The co-rotational and longitudinally displaceable coupling of the cutter feed driver with the tool barrel is accomplished in the simplest case by a pin provided in a transverse bore in the barrel and in an elongated hole in the driver.
Alternatively this coupling can also be created by providing the driver with a splineshaft section disposed corotationally but longitudinally displaceably in a hollow section of the barrel, provided with a complementary splined internal cross section.
In another embodiment of the invention, the cutter feed driver is in the form of a splineshaft-like section in its portion protruding from the barrel, which engages at its end facing the barrel in a counterbored section of the barrel which is provided with a complementary splined internal cross section, the splineshaft section being displaceable in the barrel section by the amount required for the rocking of the cutter.
In this embodiment, too, a resilient return of the cutter feed driver relative to the barrel can be devised for the purpose of simplifying the operation of the tool, by providing a compression spring within the counterbored section of the barrel such that at one end it thrusts against the forward end surface of the splineshaft section and against the bottom of the counterbored section of the barrel.
An improvement of the operation of the tool constructed in the manner of the invention is achieved by prolonging the slot that contains the cutter into the section of the barrel that is ahead of the cutter in the direction of insertion, and into a pocket for receiving the material (dust) cut by the cutter from the wall of the pilot bore. This prevents interference with the retraction of the cutter by the dust that accumulates in the slot.
This can be accomplished also by aspirating the dust from the pilot bore. In the embodiments in which there is a splineshaft coupling between the barrel and the cutter feed driver, the latter can be bored centrally through its entire length for this purpose, so that this bore will lead into the slot containing the cutter. An apparatus for the aspiration of the dust from the orifice of the bore must then be provided on the machine that drives the undercutting tool.